This invention relates to a novel method and an apparatus for operating a circulating fluidized bed (CFB) system.
CFB systems, such as circulating fluidized boilers, include a combustion chamber, having a fast (e.g. gas velocities greater than 2 m/s) fluidized bed of solid particles established therein. A particle separator is connected to a discharge opening in the upper part of the combustion chamber and a return duct is connected thereto, for recycling particles separated in the separator through a solid particle inlet into the lower part of the combustion chamber. A heat exchanger may be connected to the recycling system for recovering heat from the system.
Typically, heat is recovered from fluidized bed boilers by heat transfer surfaces in the combustion chamber and in a convection section disposed in the pat of the hot flue gas. The peripheral walls of the combustion chambers are made as membrane walls in which vertical tubes are combined by fins to form evaporating surfaces. Additional heat transfer surfaces such as superheaters may be disposed within the upper part of the combustion chamber for superheating the steam.
Corrosion and erosion constitute problems in the high temperature and high flow velocity surroundings within the combustion chamber and convection section, and the heat transfer surfaces have to be made of expensive heat resistant material.
In conventional CFB systems, it also can be difficult to achieve desired superheating of steam at low load conditions. The combustion chamber exit gas temperature tends to decrease with decreasing load and special arrangements have to be made in order to achieve desired results with superheaters in the convection section. Additional superheaters disposed within the combustion chamber are not an acceptable solution since they increase cost and control problems in the boiler. Thus there is a need, especially for pressurized applications, to find new ways of adding heat transfer surfaces into the system without having to increase the size of the combustion chamber, which would increase the size of the pressure vessel.
It has been suggested to use external separate heat exchangers (EHE) for increasing superheating capacity. The external heat exchangers usually consume too much space, making it difficult to control heat transfer at different (e.g. fluctuating) load conditions.
It has been suggested in U.S. Pat. No. 4,716,856 to include heat transfer surfaces in the recycling system of a circulating fluidized bed reactor. The heat transfer surfaces are then disposed in a fluidized bed of solid circulating material collected in a heat exchanger chamber formed in the bottom part of the return duct. Thus the circulating solid material provides the additional heat needed for superheating, etc., without a need to utilize separate external heat exchangers. This system is, however, dependent on hot solid particles being entrained with the flue gases and recycled into the heat exchanger chamber. At low load conditions, and hence low fluidizing gas flow velocities, the volume of hot particles present in the gas flow may be too small to maintain the heat transfer capacity needed for superheating.
The present invention provides a method and an apparatus for operating circulating fluidized bed systems in which the above mentioned drawbacks are minimized. The present invention also provides an improved method and apparatus for heat recovery at different loads in circulating fluidized bed systems.
According to the present invention an improved method of operating a CFB system is provided including a heat exchanger chamber formed in the lower part of the return duct, the heat exchanger chamber having one wall section in common with the combustion chamber. The method comprises the following steps: (a) Establishing a fast fluidized bed of solid particles in the combustion chamber so that a particle suspension comprising flue gases and solid particles entrained therein is caused to flow upwardly in the combustion chamber and to be discharged through the discharge opening. (b) Separating solid particles from the particle suspension in a particle separator. (c) Directing separated solid particles into a return duct having a heat exchanger chamber in a lower portion thereof. (d) Establishing a bed of solid particles in the heat exchanger chamber. (e) Reintroducing solid particles from the heat exchanger chamber into the combustion chamber through a solid particle inlet disposed in the common wall section; and (f) introducing additional solid particles directly from the combustion chamber into the lower portion of the return duct.
During high load conditions a large solid particle flow is entrained with the flue gases and recycled through the separator, return duct and heat exchanger chamber into the combustion chamber, providing the desired heat transfer capacity. During low load conditions solid particles are caused to flow directly from the combustion chamber into the heat exchanger chamber through a passage in the common wall therebetween, the direct solid flow enhancing the heat transfer capacity so that it achieves the desired level.
Solid particles may be reintroduced from the heat exchanger chamber into the combustion chamber through a solid particle inlet or inlets disposed in the common wall between the chambers. The solid particle inlet or inlets may be disposed in the lower part of the heat exchanger chamber below the surface of the particle bed therein or the solid particle inlet(s) may constitute overflow openings higher up in the heat exchanger chamber, allowing gas also to flow from the return duct into the combustion chamber. In many applications both types of solid particle inlets are utilized. The inlets disposed below the surface of the particle bed comprise--according to a preferred embodiment of the invention--a solid flow seal formed by two or more narrow (small vertical dimension) substantially horizontal slots disposed on top of each other in the common wall preventing uncontrolled flow of particles through the inlets. The slots may be prefabricated in a frame like construction which is built into the wall.
In order to prevent a direct and an uncontrolled flow of particles through the slots, the slots should have a height (h) to length (l) ratio h/l&lt;0.5. Slots having a length of about 200 mm to 300 ram, e.g. the length of the cross section of the common wall in which the slots are formed, should have a height &lt;100 mm to 150 mm to be able to prevent an uncontrolled flow of particles therethrough. In such slots solid particles tend to build up and form a sealing plug preventing flow by gravity. A desired flow of solid particles through the slots is achieved by transporting gas being introduced into the bed in the vicinity of the slots. Thus it is possible to control the solid particle flow through the inlets and the flow of particles passing the heat transfer surfaces in the heat exchanger bed.
The substantially horizontal slots in the inlets need not be completely horizontal, but rather can be inclined, having outlet ends in the combustion chamber at a higher level than the inlet ends in the return duct, so that the length (l) of the slots can be further decreased compared to horizontal slots having the same cross section. The inclined slots also prevent coarse material from accumulating at the inlet end of the slots.
The total vertical extension h.sub.tot needed for an imaginary single large opening can--according to one important aspect of the invention--be divided into several vertical extensions h.sub.1, h.sub.2, h.sub.3, ..., each divided vertical extension being just a fraction of the total h.sub.tot needed. The length (l) of each slot can then be decreased in the same proportion as the vertical extension is decreased, without the sealing effect of the solid flow being decreased.
According to a preferred embodiment of the invention short slots, only long enough to extend through a common (usually refractory) lined membrane wall, between the heat exchanger chamber and the combustion chamber, can be used for transporting particles, while still providing an adequate solid flow seal. These slots have an approximate length (l)=the total width (w) of the common wall between the two chambers, the width of the wall including tubes and refractory lining. This is a considerable improvement over prior art L-valve seals which reach far out from the combustion chamber and consume a large amount of space. The present invention provides a very compact structure in which the solid flow seal can be integrated into the wall construction.
The solid flow seal passages may easily be formed in the fins which connect the tubes in a conventional membrane tube wall. In many cases the passages may be formed in a wall section where tubes have been bent apart from each other to provide the space needed for the passages. The passages may be disposed on top of each other, forming e.g. a Ahlstrom "gill seal" type of solid flow seal connection, and combined in prefabricated frames.
The solid particles may--especially at high load conditions--be reintroduced into the combustion chamber by overflow through one or more overflow openings formed in the common wall at a higher level than the solid particle inlets described above. Especially at high load conditions, both types of solid particle inlets may be used.
By using small overflow openings for reintroducing solid particles from the heat exchanger chamber into the combustion chamber, large particles are prevented from flowing in the other direction, i.e. from the combustion chamber into the heat exchanger chamber. During high load conditions rather large particles may be fluidized in the lower part of the combustion chamber. It is not desirable to move large particles into the heat exchanger chamber.
At low load conditions there may be no need to reintroduce solid particles through an overflow opening into the combustion chamber. The bed surface level in the heat exchanger chamber may be maintained below the overflow opening and the overflow opening may instead be utilized as a passage introducing solid particles from the combustion chamber into the heat exchanger chamber. At low load conditions the reintroduction of solid particles may also take place solely through an overflow opening or through both types of solid particle inlets.
The overflow opening reintroducing solid particles into the combustion chamber may be constructed so that it can simultaneously or alternatively introduce solid material from the combustion chamber into the return duct as well as from the return duct into the combustion chamber. On the other hand different types of openings for introducing solid particles into the return duct and for reintroducing solid particles into the combustion chamber may be used. The various particle-introducing openings may be disposed horizontally side by side, or stacked vertically one on top of the other. Gas nozzles, injecting gas flows into or in the vicinity of the overflow openings, may be used to control the solid flow through the openings, e.g. to prevent solid particles from flowing from the combustion chamber into the return duct. Gas flowing through the openings may be used as secondary or similar air in the combustion chamber. Additional openings may also be formed higher up in the common wall between the return duct and the combustion chamber primarily for introducing gas from the return duct into the combustion chamber.
The bed of particles in the heat exchanger is fluidized in order to enable heat transfer between the particles and heat transfer surfaces disposed in the bed. Fluidizing gas is preferably discharged from the return duct through gas flow openings above the fluidized bed. In order to prevent fluidizing gas from flowing into the particle separator a gas seal may be disposed between the return duct and the separator. The gas seal may constitute a bed of particles disposed in a bottom duct of the separator. The bottom duct is preferably connected by a solid flow seal with the return duct. The solid flow seal preferably comprises two or more vertically narrow horizontal slot like openings stacked one on top of the other in a common wall between the bottom duct and the return duct preventing uncontrolled flow of particles from the bottom duct into the return duct.
The heat transfer from particles to heat transfer surfaces, such as superheater surfaces, may--according to a preferred embodiment of the invention--be controlled by fluidizing gas. An increased fluidizing gas flow and increased movement of particles around the heat transfer surfaces provides increased heat transfer. Gas, such as air or inert gas, for heat transfer control may be introduced through several separate nozzles.
According to another preferred embodiment of the invention, heat transfer may be controlled by controlling the flow of hot solid particles flowing through the bed, i.e. from the bed surface down to the solid particle inlets in the bottom of the bed. This is achieved by controlling the flow of transporting gas controlling the reintroduction of particles through the solid particle inlets. A surplus portion of solid particles being introduced into the return duct and not needed for heat transfer are reintroduced into the combustion chamber by overflow.
By decreasing the amount of solid material being transported through the solid particle inlet(s) below the bed surface and correspondingly increasing the overflow of particles into the combustion chamber, an increased volume of particles reaches only as high as the surface of the bed of solid particles before being reintroduced into the combustion chamber. A decreased volume of solid particles thus flows through the bed in contact with heat transfer surfaces. Thus the temperature in the bed decreases, and heat transfer also decreases due to the lower temperature difference between particles and heat transfer surfaces.
By increasing the volume of solid material being transported through the solid particle inlet(s) an increased amount of fresh hot solid material may continuously be transported through the bed, increasing the temperature and thus increasing heat transfer in the bed.
In the bottom of the return duct the bed moves slowly downwardly as solid material is reintroduced into the combustion chamber and new material is continuously added on top of the bed. The height of the bed may thus--according to a third preferred embodiment of the invention--be controlled, in a heat exchanger chamber not having an overflow opening, by controlling the transporting gas reintroducing solid material into the combustion chamber. The height of the bed may then--in some cases--be used to control the heat transfer.
A barrier bed portion of solid particles may be maintained between transporting gas inlets in the vicinity of the solid particle inlets below the bed surface and the fluidizing gas inlets in the heat transfer section of the heat exchanger chamber. A barrier bed close to the transporting gas inlets prevents transporting gas from interfering with the desired heat transfer, while a barrier bed portion of solid particles maintained in the heat transfer section prevents fluidizing gas from interfering with the transport of solid particles through the bed. In most cases both these goals can be achieved using a single barrier bed portion.
The heat exchanger chamber may have an inclined or staged bottom in order to more easily provide for a suitable barrier bed between the transporting gas inlets and fluidizing gas inlets. A partition wall may be disposed on the inclined bottom between the heat transfer and particle transporting sections. Fluidizing gas is introduced through the upper inclined bottom portion into the heat transfer section. Transporting gas is introduced through the lower portion of the inclined bottom. A barrier bed of, for example, only slightly fluidized particles is maintained preferably on the lower portion of the inclined bottom.
The present invention may be applied in reactor systems having return ducts with horizontal bottoms also, as long as care is taken that a barrier bed is allowed to be formed on a portion of the bottom, for preventing transporting gas or fluidizing gas from interfering with each other.
The heat exchanger chamber may--according to another aspect of the present invention--have a staged bottom, in which a heat transfer section and solid particle inlets are disposed at different levels. The heat transfer section is disposed at a higher level than the discharge of solid particles. The solid particle inlets may open into a downwardly directed duct or channel portion of the heat exchanger chamber, the channel portion being connected to the combustion chamber.
The particles are preferably reintroduced directly from the heat exchanger chamber into the combustion chamber, but can--if necessary--be reintroduced through an intermediate chamber, which then is connected with the combustion chamber.
The present invention provides an important improvement in the control of heat transfer at both high and low loads. A large enough flow of solid material is maintained in the heat exchanger chamber both during high and low load conditions to achieve the desired heat transfer capacity.
The gas space in the heat transfer zone contains primarily clean fluidizing gas without alkaline, chlorine, or other corrosive gaseous components, and thus provides very advantageous conditions for superheating. Superheaters in this zone may thus be heated to much higher temperatures than possible in corrosive conditions prevailing in the combustion chamber itself. Steam of &gt;500.degree. C., even &gt;550.degree. C., may also be produced when burning corrosive gaseous component-containing fuels.
It has especially been a problem in waste/RDF burning boilers to utilize the heat for superheating, due to the unclean gases, containing different kinds of corrosion causing components. The present invention overcomes this problem by providing a system in which superheater surfaces contact hot circulating material in a safe gas atmosphere. Also erosion is minimized by using a slowly bubbling bed (having gas velocities of &lt;1 m/s) in the heat exchanger chamber. Particles colliding with the heat transfer surfaces have a very low impact velocity. Additionally, erosion in the return duct bed is relatively low due to the small particle size of bed material.
According to the invention when the bed in the solid bed chamber is divided into a heat transfer section and a solid particle discharge section close to the solid particle inlets by an inclined bottom (or a separate lower outlet channel portion in the return duct) a significant advantage is achieved; namely, large particles (e.g. ash particles, agglomerates formed in the bed, or refractory material broken loose from the return duct walls) fall by gravity downwardly in the return duct below the level of the fluidizing gas inlet, and away from the heat transfer zone, where they could cause mechanical damage and other problems (such as a decrease in heat transfer).
The present invention provides a very simple and compact CFB boiler construction. The whole recycling system, including separator and return duct, may be primarily constructed of two at least partly parallel vertical water tube wall panels forming a substantially vertical channel therebetween. The channel preferably has one wall in common with the combustion chamber. The channel typically has a separator in its upper part, a return duct in its middle part, and a solid bed chamber in its lowermost part. The solid inlets, overflow openings, and other gas and solid material passages connecting the return duct with the combustion chamber may be prefabricated in the common wall,e.g. as a frame like construction. Such a frame structure may also be easily connected to the membrane wall on site.
The present invention is particularly advantageous in pressurized fluidized bed systems (i.e. operated at significantly above atmospheric pressure, e.g. at least twice atmospheric pressure), as additional heat transfer surfaces can be located in the return duct in a usually empty space in the pressure vessel and as the additional heat transfer can be controlled by relatively small gas flows and hence small equipment. The present invention provides a compact combustor system, which is easy to build into a pressure vessel.